In the past, electromagnetic radiation has been used for the detection and identification of objects buried in the ground. Early uses of various types of radiation for detecting subsurface phenomena are quite varied.
Seismic studies have used sound. Sound travels at different speeds in different mediums. The time of travel of sound has been used to determine subsurface characteristics. For example, a series of vibration or sound detecting sensors can be strategically located on the surface below which the characteristics are to be determined. A solid plate can be placed in the vicinity of the sensors. The plate can be impacted to transfer energy into the subsurface medium. The reflection of the energy by different subsurface material causes the sensors on the surface to receive reflected signals at different times. The time differences received by the surface sensors are analyzed to determine the characteristics of the subsurface medium.
The military has detected mines by propagating radiation into the earth and analyzing the reflected radiation. Also, identification schemes have been devised to try to determine the characteristics of buried objects. Object identification systems use empirical data to evaluate the subsurface object. Systems have been devised for determining the location, size and configuration of subsurface objects. However, there are typically many constraints placed on the type of subsurface objects that can be detected. For example, the subsurface object may need to be metal.
Most prior technology is directed to detecting a solid foreign body embedded in another medium. The present system and method seeks to determine information about the characteristics of the medium itself as well as characteristics of the foreign body or void in the other medium.
Typical systems and methods are inappropriate in specific situations for determining many of the conditions within a medium. Delamination in a subsurface medium is a problem which is not easily detected using currently available technology. Delamination occurs when layers composed of two different substances separate. Since the layers remain abutting, it is extremely difficult to detect the separation. Debonding is another phenomenon that occurs within a medium which is difficult to detect using current technology. Debonding occurs when a single, monolithic layer separates to form two layers. Debonding can be extremely difficult to detect. Also, stripping has proved to be an extremely difficult phenomenon to detect. Typically, stripping is associated with asphalt roads. Stripping is the breaking of the adhesive bond between the aggregate surface and the asphalt cement. Usually, stripping begins at the bottom of the asphalt layer and moves upward until the pavement structure is weakened. Cracks appear in the pavement and, in advanced stages, the pavement begins to disintegrate. Each of the phenomenon discussed above, i.e., delamination, debonding and stripping, are extremely difficult or impossible to determine using present technology.
The term ground-penetrating radar represents a wide range of applications and types of radio detection and ranging (RADAR) equipment. Ground-penetrating radar equipment varies from ground coupled to air coupled systems with frequency ranges from 10's of megahertz to 3-4000 megahertz. Applications for the technology generally exit in the shallow geophysical industry, pavement and civil structures area, and in utility location.
A radar unit is a device that emits a pulse of electromagnetic energy and is able to determine the presence or absence of a target by examining the reflected energy from that pulse. Preferably, the pulse is short. By way of example, if a pulse is fired into a pavement structure, the electromagnetic wave travels until it meets with a dielectric discontinuity. The dielectric discontinuity may be caused by many things, for example, a change in pavement layer, moisture within or beneath the layer, an air void, or some other change in the dielectric constant of the material in the path of the wave. A portion of the wave is reflected by this discontinuity and a portion continues to travel through the second medium.
The amount of energy reflected at the discontinuity is a function of the wave impedance of the two materials. At the interface between materials with similar dielectric properties, such as two lifts of an asphalt concrete pavement, most of the energy passes through the interface and very little is reflected back to the transmitter. Conversely, where the difference in dielectrics is significant, such as in an asphalt layer over concrete or a structural layer over base course, more of the energy is reflected to the transmitter.
This reflection phenomenon is the theoretical basis for the production of various radar signatures produced by different subsurface anomalies.
The ability of short-pulse ground-penetrating radar ("GPR") to detect, locate, and characterize subsurface anomalies is well established and well documented. Previous research has shown that GPR is an effective tool for locating voids, identifying stripping in asphalt layers, determining the presence of moisture (at various levels in the pavement structure), identifying areas of delamination in overlays, and providing useful information on other areas of subsurface problems and discontinuities and thickness of individual layers.
Ground-penetrating radar has been used for determining soil interfaces, bedrock profiling, detecting soil water movement, depth of permafrost, evaluation of fractures, thickness of peat and ice evaluation of dam safety and in coal mining. Such deep investigations (40-160') usually involve the use of lower frequency systems than the present invention. In the 6-30' range, mid-range frequency radars are used to locate underground storage tanks, pipelines (water, sewage, gas electrical, etc.), buried drums, archeological applications, and sinkholes. High frequency ground-penetrating radar has been used by the military to determine the location of plastic mines and other unexploded munitions.
It is important to understand the basic radar reflection phenomenon, the relationship between the radar timing and the depth within the media, as well as the effect of anomaly thickness and anomaly location.
A simplified model of the interaction of impinging electromagnetic waves with a dielectric discontinuity can be readily expressed. A portion of the incident electromagnetic wave may be reflected by this discontinuity and a portion continues to propagate down into a second medium. The proportion of the incident wave that is reflected at the interface is determined by the reflection coefficient, P, associated with the boundary. This reflection coefficient is given by: ##EQU1## where n.sub.1 and n.sub.2 are the wave impedance of medium 1 and 2, respectively. In general, the wave impedance is a complex number, but simplification is possible for certain groups of materials. For example, the wave impedance for a highly conductive material such as silver or copper is essentially zero, whereas that for a non-ferrous, non-conducting material such as dry concrete or soil is given by: ##EQU2## where .mu..sub.0 is the permeability of free space (a constant), .epsilon..sub.0 is the permittivity of free space (a constant), and .epsilon..sub.r is the relative dielectric constant of the material in question. Since .epsilon..sub.r is the only variable in the wave impedance expression in Equation 2, the reflection coefficient expression shown in Equation 1 can be reduced as follows for interfaces between nonferrous, nonconducting materials: ##EQU3## where .epsilon..sub.r.sbsb.1 and .epsilon..sub.r.sbsb.2 are the relative dielectric constants for media 1 and 2, respectively.
Note from Equation 3 that if medium 1 has a smaller relative dielectric constant than medium 2, P has a negative value. On the other hand, if medium 1 has a larger relative dielectric constant than medium 2, P is positive. Furthermore, the magnitude of P is proportional to the difference between .sqroot..epsilon..sub.r.sub.1 and .sqroot..epsilon..sub.r.sbsb.2. Thus, at an interface between materials with similar dielectric properties, most of the impinging wave energy passes through the interface and little is reflected back toward the transmitting source. On the other hand, a large reflection and a correspondingly small transmission occur at an interface between two materials with greatly different relative dielectric constants. This reflection phenomenon is the theoretical basis for the production of specific "signatures" by various subsurface objects and features.
Table 1 lists approximate .epsilon..sub.r values for selected materials. Table 1 is intended as illustration only because many of the listed materials, .epsilon..sub.r can vary significantly from the listed typical value(s), depending upon a number of factors. Note that .epsilon..sub.r is always greater than or equal to 1. In general, it is safe to assume that virtually all materials encountered in a typical ground-penetrating survey application will have relative dielectric constants which lie between 1 and 81.
TABLE 1 ______________________________________ TYPICAL RELATIVE DIELECTRIC CONSTANTS FOR SELECTED MATERIALS MATERIAL .epsilon..sub.r ______________________________________ Air 1 Pure Water 81 Seawater 81 Freshwater ice 4 Seawater ice 6 Snow (firm) 1.4 Sand (dry) 5 Sand (saturated) 30 Clay (saturated) 10 Granite (dry) 5 Granite (wet) 7 Limestone (dry) 7 Limestone (wet) 8 Shale (wet) 7 Sandstone (wet) 6 Soil sandy dry 2-4 sandy wet 20-25 loamy dry 2-6 loamy wet 15-20 clay dry 2-6 clay wet 10-20 Permafrost 6-13 Strong concrete dry 5-9 soaked 20 hrs 10-15 Cracked concrete dry 4-5 soaked 20 hrs 13-20 Asphalt 12-16 ______________________________________
In addition, the depth of a discontinuity below the surface can also be determined from the relative timing of the received reflection. The relationship between the depth of a discontinuity and the timing of the reflected signal can be described. The electromagnetic wave travels from an antenna (transmitting mode) to the interface and back to the antenna (receiving mode) in a time t. The total distance traveled by the wave in this time is 2d. Furthermore, from electromagnetic wave theory, one can determine that provided a material #1 is electromagnetically linear, homogeneous, isotropic, and nonmagnetic; the speed by which the wave propagates, u.sub.p, in the material is given by: ##EQU4## where c=the speed of light in free space (c=3.times.10.sup.8 m/sec) and .epsilon..sub.r =the relative dielectric constant of material #1. The speed is given by the total distance traveled divided by the time required to traverse the distance, thus the equality: ##EQU5## Solving the equality for the depth, d, at which the interface occurs yields the expression: ##EQU6##
There are devices that provide some information about the conditions within a medium. Most of these devices require that the radiation emitting device be in contact with, or in very close proximity to, the surface under which the measurements are to be taken. Devices that require a direct contact with the surface under investigation greatly inhibit the accuracy of the data and the speed with which the data can be acquired. For example, the requirement that the device be in direct contact with, or in very close proximity to, the surface causes structural wear on the device and limits the types of terrain or surface over which the device can be used and inhibits the rate at which the data can be taken.
There is, thus, a need for an medium measurement system and method which provides information about the conditions within a medium without the mechanical and logistical restrictions of currently available devices, which, at the same time, provides nondestructive measurements of the medium in question, and which provides measurements without directly contacting the medium under investigation.
Recognizing the need for an improved system and method for the measurement of medium characteristics, it is, therefore, a general feature of the present invention to provide a novel system and method for making measurements within a medium, which is nondestructive to the medium, and which operates without direct contact with the medium under investigation.
It is therefore a feature of the present invention to provide a medium measurement system and method for efficiently detecting the thickness of a medium or the thickness of multiple mediums.
Similarly, it is a feature of the present invention to provide a medium measurement system and method for efficiently detecting voids, locating voids and determining the size of voids in a medium.
It is a primary feature of the present invention to provide an apparatus for determining subsurface medium characteristics having an integrated, monocoque microwave unit which contains all the microwave components within a rigid structure wherein the structural integrity is in the microwave unit itself rather than a frame, support or the like.
Also, it is a feature of the present invention to provide an integrated, monocoque microwave unit which requires no connection or disconnection of microwave components, cables or parts such that clutter is maintained at a minimum.
It is also primary feature of the present invention to provide an apparatus for determining subsurface medium characteristics having an integrated, monocoque microwave unit which includes a monocycle pulse generator that uses at least one planar waveguide structure for generating a monocycle pulse.
It is a feature of the present invention to provide an apparatus for determining defects associated with non-metallic materials.
Also, it is a feature of the present invention to provide an apparatus for evaluating bulk material.
Still further it is feature of the present invention to evaluate fiber glass.
Yet still further it is a feature of the present invention to evaluate concrete and asphalt-type material.
It is another feature of the present invention to provide a medium measurement system and method for determining the delamination of two different layers within a medium.
Yet another feature of the present invention is to provide a medium measurement system and method for detecting the debonding of a single, monolithic layer within the medium.
Still another feature of the present invention is to provide a medium measurement system and method for determining stripping within an asphalt medium so that the breaking of the adhesive bond between the aggregate surface and the asphalt cement can be detected and the asphalt can be readily repaired.
Yet still another feature of the present invention is to provide a medium measurement system and method for determining the thickness of one or more layers within a medium.
Further, a feature of the present invention is to provide a medium measurement system that is extremely mobile. The present invention provides for ruggedized equipment that can be mounted on and transported by a vehicle for readily determining the medium conditions in the vicinity of the vehicle.
Another feature of the present invention is to provide a medium measurement system and method which continuously determines a plurality of simultaneous measurement paths for the accurate detection, location and sizing of voids, of delamination, of debonding, of stripping, and of the various layers of the medium.
In association with the noted features of the present invention examples may be roof evaluation, road evaluation, the evaluation of non-metallic solids, bridge deck evaluations, storm sewer evaluations, subway tunnel evaluations, aqueduct tunnel evaluations, and layered fiber glass material evaluations.
Additional objects and advantages of the invention will be set forth in part in the description which follows, and in part will become apparent from the description, or may be learned by practice of the invention. The features, objects and advantages of the invention may be realized and obtained by means of the instrumentalities, combinations and steps particularly pointed out in the appended claims.